Individuals with neurofibromatosis type 1 (NF1) carry mutations in the NF1 tumor suppressor gene and develop benign peripheral nerve sheath tumors (neurofibromas) that cause significant morbidity, and mortality if they compress vital structures, and which can transform to malignant peripheral nerve sheath tumors (MPNST). Neurofibromas contain normal nerve constituents: axons, Schwann cells, fibroblasts, macrophages and mast cells, and aberrant Schwann cells free of axons. Tumorigenesis results from complete loss of function at NF1, as neurofibroma Schwann cells are characterized by biallelic mutations in NF1, with other cell types recruited secondarily. We developed mouse model systems that are accurate mimics of neurofibroma formation in NF1, and performed large scale gene expression array analyses to identify candidate genes relevant to neurofibroma formation. Our robust Preliminary Data shows that a chemokine receptor, Cxcr3, is crucial for neurofibroma formation. In this application, we propose to use unique mouse models, neurofibroma Schwann cell precursors and Schwann cells to define the chemokine pathways through which Nf1 loss in Schwann cells leads to the presence of Cxcr3+ cells in neurofibroma (Aim 1) and to define CXCr3-expressing cells in neurofibroma and test their function (Aim 2). Finally, we will define molecules that are overexpressed in neurofibroma Schwann cells and are necessary for their survival (Aim 3). Together these studies will identify cellular and molecular underpinnings of tumor formation in the nervous system, and identify therapeutic targets for the treatment of NF1.